Senior security minister Djoko Suyanto said he believed Islamic militants had been plotting an attack ahead of Easter celebrations, and the U.S. embassy urged Americans to be vigilant.

"The army and police are under high alert," he told reporters, adding that troops would be deployed at churches and other strategic locations. "We want to guarantee safety."

Indonesia, the world's most populous Muslim nation, has been battling extremists since 2002 when al-Qaida-linked militants attacked two nightclubs on Bali island, killing 202 people, many of them foreign tourists.

There have been several attacks since then targeting glitzy hotels, restaurants and an embassy, killing another 60. Hundreds of suspects have been arrested, convicted and thrown in jail.

In recent months, small bands of militants hoping to turn the secular nation of 237 million into an Islamic state have shifted their focus to local "enemies."

National Police Chief Gen. Timur Pradopo said the 19 suspects were arrested Thursday, including six accused in a series of mail bombs sent last month to liberal Muslim activists and a former anti-terror chief.

Four people were wounded in the parcel bombings, none seriously.

The arrested men eventually led police to the gas pipeline 100 meters (yards) from a Catholic church large enough to hold 3,000 people in Serpong, where they found the massive, high explosive bomb.

Suyanto said he believed the militants had been planning to disrupt Good Friday celebrations.

The U.S. embassy in Jakarta issued a statement urging Americans to be especially cautious over the weekend and to stay clear of demonstrations.

"Even demonstrations intended to be peaceful can turn confrontational and possibly escalate into violence," it said.

Ninety percent of Indonesians are Muslim, though most practice a moderate form of the faith and abhor violence. A small, extremist fringe has become more vocal, and violent, in recent years.